Screens of the abovementioned category have been known for a long time. The invention will be illustrated below by referring to a specific field of application for such screening devices, namely the pulp industry and in this regard particularly in connection with the washing of pulp.
Through U.S. Pat. No. 5,027,620 (Kamyr) a screening device of this type is already known which comprises a cylindrical housing with: an inlet and an outlet for pulp, a movable supporting member which supports hollow screening members, means for vertical movement of the said supporting member, a container which is arranged on the said supporting member, which container is open at both ends and has a movable piston member arranged inside it together with means for moving the said piston member. In this known screening device, the drive unit for the vertical movement of said piston member, in the form of an hydraulic piston unit, is arranged inside the housing of the screening device, for moving the piston member. However, this arrangement has been found to entail certain disadvantages, since it has been found difficult completely to avoid leakage of hydraulic fluid, which has a negative effect on the quality of the pulp. Consequently a goal is to attempt to eliminate completely the risk of such leakages.
The solution disclosed in U.S. Pat. No. 4,971,694 (Kamyr) solves the latter problem by placing the hydraulic unit entirely outside the housing of the screening device. However, problems of another type have been found to arise with a device of the latter type. These problems are more related to the design of the apparatus and are due to the fact that the drive unit, which moves the piston member, exerts very large forces. This is because, in such a screening device, relatively large liquid volumes (about 1 m.sup.3) have to be moved within relatively short times (about 2 sec). Consequently, rigid structures are needed for mounting the hydraulic unit. In a device according to U.S. Pat. No. 4,971,694, this structure consists of a stout beam which, simply for this reason, has been arranged straight through the housing at its upper end. Despite its relatively generous dimensions, the beam bends markedly when the hydraulic unit is operating, which can be found to be troublesome. For various reasons, therefore, this latter solution cannot be considered to be ideal.